Thesaurus.com
Dictionary.com
Definitions

efficacious

[ef-i-key-shuhs] / ˌɛf ɪˈkeɪ ʃəs /
ADJECTIVE
effective, productive
Synonyms
STRONG
effective effectual productive
WEAK
active adequate capable competent efficient energetic influential operative potent powerful puissant serviceable strong successful useful virtuous
Antonyms
STRONG
impotent incapable unproductive useless weak
WEAK
inefficacious inefficient unsuccessful

Usage

What are other ways to say efficacious? The adjective efficacious suggests the capability of achieving a certain end: an efficacious plan, medicine. Effectual is used especially of that which produces the effect desired or intended, or a decisive result: An effectual bombardment silenced the enemy. Effective is applied to a person or a thing that has the power to, or which actually does, produce an effect: an effective boss, remedy, speech. Efficient (applied also to persons) implies the skillful use of energy or industry to accomplish desired results with little waste of effort: efficient methods; an efficient manager. 

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

Lilly is also developing a highly efficacious injection called retatrutide that targets GIP and GLP-1 — both of which are activated by tirzepatide — in addition to glucagon.

From MarketWatch • Apr. 1, 2026

Instead of manipulating government for their own narrow interests, the rich can add a distinctively independent-minded, public-spirited and efficacious set of voices to the national debate.

From The Wall Street Journal • Feb. 17, 2026

“If something was to be made available and shown to be efficacious and safe, I think there would be wide use of it,” Jonker says.

From Science Magazine • May 20, 2024

But there comes a point where, if that’s not efficacious, you’re going to have to do something more publicly in the courtroom.

From Salon • Feb. 2, 2024

One of the best and most efficacious remedies would be for the person who has taken possession of them to go there to live.

From "The Prince" by Niccolò Machiavelli